1. Field of the Invention
The present invention relates generally to a pulse generator generating a short pulse having high voltage and a large amount of current. Specifically, the present invention relates to such a pulse generator including a semiconductor switch and a saturable transformer.
2. Description of the Background Art
Generally, apparatus for pulse laser excitation, pulse plasma generation or pulse denitration, is well known as a pulse generator. In order to obtain a desired pulse, discharging switches are used, such as thyratron and trigatron switches, to directly switch high voltage and a large amount of current. A combination of a semiconductor switch and a saturable reactor which functions as a magnetic switch is also utilized.
Additionally, one saturable transformer for raising voltage concurrently with pulse compressing to reduce the load of a switching device is also utilized for pulse generation.
FIG. 5 illustrates a conventional circuit including a saturable reactor. A wave form generated from the circuit is also illustrated in the figure. A plurality of thyristors TH, which function as a semiconductor switch at high voltage, are connected in series. A condenser C.sub.11 connected to the circuit is charged by high voltage. A reactor L is located at the upstream of the condenser C.sub.11. A first current I.sub.1 is generated by a short-circuit from the series circuit including the condenser C.sub.11 and the reactor L. Polarity of the condenser C.sub.11 is inverted thereby to generate voltage twice as that obtained from LC inversion via the series connection of condensers C.sub.11 and C.sub.12. A second current I.sub.2 of pulse compressed is generated by this high voltage, at a circuit including a saturable reactor SR.sub.1, condensers C.sub.11, C.sub.12 and C.sub.2 which are connected in series. Then, A third current I.sub.3 of pulse compressed is generated by the voltage of the condenser C.sub.2, at a circuit including a saturable reactor SR.sub.2, condensers C.sub.3 and C.sub.2 which are connected in series. Finally, a compressed pulse I.sub.4 having large amount of current and high voltage is generated at the circuit including a saturable reactor SR.sub.3 and a discharge tube DT.
However, in the conventional pulse generator including discharging switch devices, the lifetime of the switch devices is very short, i.e., 10.sup.7 to 10.sup.8 pulse. Therefore, when the circuit is operated repeatedly, duration of the switch devices is limited.
Furthermore, in the conventional pulse generator including the combination of the semiconductor switch and the saturable reactor, the withstand voltage of the switch devices is smaller than that of the discharging switch device. Therefore, a plurality of semiconductor switches must be connected in series in order to generate high voltage. This causes the switch circuit to be enlarged because a drive circuit and a snubber circuit are added to each switch device corresponding to the number of series connection. In addition, voltage shared by respective switch becomes unequal if switches timing is upset. On the other hand, the switches may be connected in parallel, however, excess current flows through the switch if switch timing is upset or any one of switches is failed. This causes other switches to be failed.
Utilizing a pulse transformer for voltage raising to reduce the number of the switch device, and reducing the switching voltage to about 1/2 of that of output voltage by a LC inversion circuit are both available, however, the number of the switch device and the number of magnetic pulse compressed are not minimized. This enlarges the circuit compared with that including the discharging switch.
Minimization of the circuit may be done by utilizing single saturable transformer to unite function of voltage raising and magnetic pulse compression. However, the number of turns of the secondary winding of the saturable transformer becomes larger to raise the voltage raising ratio because one transformer unites two functions. This increases inductance causing LC resonance time during pulse compression to be lengthened. Thus, generation of a short pulse is limited. On the other hand, the turns ratio of the winding must be reduced to decrease inductance at the secondary area of the saturable transformer. This causes voltage raising ratio to be reduced and load on the switch device to be increased.